Hi Giorgio,
It can be a little bit of a grey area. If your trip was predominantly for personal reasons and you carried out a little business while there, it would be wise not to claim travel costs. However, if the purpose of the trip was for business and you extended it for a short time afterwards, this should be okay.
You’d need to only claim the portion of your costs associated with business travel. Your personal days there would not be claimable.
As with any expenses, you should ensure you have receipts. You don’t need any specific proof other than that, but emails or contracts can be a good way to back yourself up if you feel unsure.
Thanks,
Katie